This invention relates to a noise eliminating system, and more particularly, to a noise eliminating system suitable for eliminating pulsive noises from a discriminator in an FM stereo receiver.
The system for eliminating noises from an FM receiver is typically connected to a detection output, and during the period when a pulsive noise signal exists, the system switches a signal path from on-state to off-state to eliminate the noise signal. Such system functions as a low-pass filter during a period of 10 to several tens of micro seconds where a pulsive noise exists within an FM modulated frequency band, and so, the low-pass characteristics decreases the level of a 19 KHz pilot tone signal and a 38 KHz stereo sub-signal to thereby attenuate the amplitude of these two signals. To avoid such attenuation, the conventional system has provided therein a compensating circuit for retaining peaks at 19 and 38 KHz (P1 and P2 in FIG. 1) while the noise exists. FIG. 1 is a frequency characteristic curve showing the relationship between response and frequency of another conventional noise eliminating system provided with such compensating circuit; the solid line represents the characteristic curve in the case where no noise exits and the broken line represents that for the case where the noise is being eliminated.
However, for the very reason that the compensating circuit is used to provide the frequency characteristic at the time of noise elimination with peaks at 19 KHz and 38 KHz, the conventional system fails to eliminate 19 KHz and 38 KHz noise components. Hence, the effect of the conventional noise eliminating system has not been fully satisfactory.
It is therefore one of the objects of this invention to provide a system for eliminating a pulsed noise which is capable of eliminating every component of a noise signal while at the same time ensuring retention of the level of every stereo signal component.